The relevant science here is not physics, but neuroscience. In that, the Stoic philosophers that you cite are wrong: the psychological present cannot be infinitely divided but has a definite duration of 200 milliseconds (ms), or one-fifth of a second. Events that are separated by shorter times are perceived as happening simultaneously. We live our lives as a movie with 200 ms frames. Incidentally, that is why we perceived movies as a continuous action and not as the sequence of still frames that they really are. The other relevant concept in neuroscience is ‘extended consciousness’, a term created by Antonio Damasio to describe the uniquely human ability to perceive ourselves as beings with a past and a future. Animals live in the ‘here and now’, humans don’t. Why should we give us something that is uniquely human?
From the personal standpoint, regretting the past and worrying about the future seems like a wise thing to do. Otherwise, how can we learn from past mistakes and plan for the future? If we want to act wisely for the benefit of our community, as Stoics want to do, we have an obligation to make our actions effective. I think the goal should be to blunt the suffering that regrets and worries cause us, but not to eliminate regretting and worrying altogether. In other words, we need to learn to regret and worry in a mindful way. Neuroscience also tells us that it is not possible to completely dissociate the emotions of regretting and worrying from learning from past mistakes and planning for the future, because emoting and cognition are inseparable.
Al things considered, I regard being ‘here and now’ as something I would do from time to time as a mindfulness exercise, but not a mental state in which I want to live constantly. I also think that trying to reject regret and worrying, just like trying to reject any other emotion, is not a wise thing to do. Rather, I would look at them in a mindful way and try to see what they can teach me. As I learned from Zen practice, they will pass if I let them pass.